1. Field of the Invention
The invention relates to an image processing apparatus and image processing method for, for example, superposing another additional sub information (security information or the like) on color image information (a human face image or the like) as main information.
The invention also relates to an image forming apparatus and image forming method for forming color image information, on which sub information is superposed using the image processing apparatus and image processing method, on a recorded material.
Furthermore, the invention relates to a recorded material created using the image processing apparatus and image processing method.
Moreover, the invention relates to an image processing apparatus and image processing method for reproducing sub information from a recorded material created using the image processing apparatus and image processing method.
2. Description of the Related Art
In recent years, along with digitization of information and prevalence of the Internet, techniques such as a digital watermark, digital signature, and the like have gained recognition so as to prevent forgery and falsification of images. The digital watermark technique embeds additional sub information (sub image information) in main information (main image information) in an invisible state. This digital watermark technique has been proposed as measures against illicit copy, forgery, and falsification of personal authentication media such as ID cards and the like, and photos embedded with copyright information.
For example, Jpn. Pat. Appln. KOKAI Publication No. 9-248935 has proposed a digital watermark insertion method which embeds data in image data to be output to a printing material using the characteristics of high spatial frequency components and color difference components, which are hard to be perceived by humans.
Also, Jpn. Pat. Appln. KOKAI Publication No. 2001-268346 has proposed a printing apparatus of a digital watermark which can be confirmed by an optical filter.
These digital watermark embedding processing methods can embed sub information (sub image information) in main information in an invisible state without any deterioration of image quality using:
(1) human visual characteristics
tone discrimination capability lowers with increasing frequency of an image
color difference information is harder to discriminate than luminance information
(2) complementary color relationship example: red+cyan=achromatic color (white) (in case of additive color mixing)
(3) high-frequency carrier pattern image+complementary color relationship and color difference information
As the example of (2), red and cyan (=green+blue) have a complementary color relationship in case of additive color mixing, and even neighboring red and cyan are hardly discriminated by the human eye and are observed as an achromatic color.
As in the example of (3), a high-frequency carrier pattern image is formed by a color difference grid pattern on which red- and cyan-rich pixels are repetitively allocated, and which exploits the human visual characteristics like that their delicate color difference cannot be discriminated by the human eye and the color difference amount is determined to come out even.
If authenticity determination of a recorded material on which an image is recorded by applying the aforementioned digital watermark technique is required, sub information recorded on the recorded material is reproduced using key information, and the authenticity is determined based on the reproduction result of the sub information, as described in the above two references.
More specifically, an input device such as a scanner, camera, or the like reads composite image information embedded with sub information as color difference information. Next, an image processor reproduces the sub information from the composite image information using a digital filter having frequency components according to specific frequency information of key information, and then checks authenticity based on the reproduction result.
However, the aforementioned technique suffers the following problems. That is, an electrophotography method is adapted as the recording method for a general digital copying machine and the like. The electrophotography method allows fast printing that can record even color images at several ten ppm or more. The electrophotography method forms a color image using four color toners, i.e., yellow, magenta, cyan, and black. Upon forming a color image using these four colors, subtractive color mixing that expresses a color image by superposing pixels of respective colors can be used. However, it is difficult in case of fast printing to accurately superpose respective color pixels. If respective pixels are not accurately superposed, color misregistration occurs.
To solve this problem, for example, the following method is known. That is, the arranging direction of a plurality of magenta pixels, that of a plurality of cyan pixels, and that of a plurality of black pixels are given tilts of different angles with respect to that of a plurality of yellow pixels. Then, these pluralities of yellow, magenta, cyan, and black pixels are composited to express a color image. As a result, a color image can be formed without accurately superposing inks. Hence, color misregistration upon deviation of pixels can be prevented.
However, the arranging direction of a plurality of pixels which form the color difference grid pattern does not correspond to those of the pluralities of magenta, cyan, and black pixels. Hence, when this color difference grid pattern is modulated by sub information, and color image information superposed with the modulated color difference grid pattern is printed by the electrophotography method, the color difference grid pattern is disturbed. That is, the sub information cannot be appropriately superposed on the color image information.
By contrast, Jpn. Pat. Appln. KOKAI Publication No. 2004-328217 has proposed a method of superposing a color difference grid pattern modulated by sub information on main information formed by alternate driving that alternately forms pixels for respective recording lines. This reference discloses a method in which after sub information is embedded in respective pieces of RGB image information, which have a tilt of a predetermined angle in a predetermined direction, the respective pieces of RGB image information embedded with the sub information are given a tilt of a predetermined angle in a direction opposite to the predetermined direction so as to alternately form pixels for respective recording lines. However, with this method, all R, G, and B colors have a common tilt angle, and pixels are formed by alternate driving. For this reason, this method cannot solve the aforementioned problem in a screening method in which the arrangement directions of pluralities of color pixels are different.